Wet sanding takes very little time. If the existing paint has a lot of old defects(scratches, dings, paint application imperfections)... I would use some 400 3M Wet/Dry - wet to remove all the imperfections. You do not need to remove 100% of the old paint unless the old/new paints are imcompatable. I would then lightly sand out any 400 sized scratches with 600 3M Wet/Dry. Then just prime/sand/paint as normal. Good luck!